If you used the Journal in the app, you should tap the "Export My PDF" button, then tap the "Action" button in the bottom right corner of the toolbar and export the PDF to iBooks or email it to yourself.

Once you have done this, you have all the data that was saved in the app and you can then reset the app (it resets and deletes all the data so it is important you do the steps above and confirm you emailed yourself all the data and PDF before doing the next step below).

If you confirmed you have all your data you sent yourself, you can then go to the app's Settings > Reset Sprout and tap the Reset Sprout button. You can then add your new details to the Settings area of the app including your due date and then you can use the app for your new pregnancy.